This invention relates to an all-pass circuit which may be described more particularly as an automatically variable all-pass phase equalizer circuit.
In digital transmission systems, such as telecommunication systems, the cable length is too long for end-to-end signal transmission without equalization, amplification and regeneration. Regenerators are interposed along the cable to offset the effects of attenuation, as a function of frequency, in the cable conductors. Typically regenerator circuitry includes an automatic line buildout (ALBO) circuit. The function of the ALBO circuit is to add shaped loss, if required, to the cable section so that the regenerator fixed gain equalizer can compensate for a predetermined loss shape. If the cable section is short, most of the loss shape is provided by the ALBO circuit. For longer cable sections, the ALBO adds less and less shaped loss to the incoming signal.
In the longer cable lengths in which the ALBO has minimum effect, the cable pair introduces excessive phase distortion that cannot be compensated by a fixed equalizer. The resulting phase distortion causes a performance degradation that can be minimized by an all-pass circuit which is inserted in tandem with the ALBO circuit to impart phase equalization to the equalized signal.
One type of all-pass circuit is an active filter in which attenuation is unchanged throughout the desired frequency range but phase equalization is provided to complement the phase distortion of the long cable length throughout the desired frequency range. One prior art all-pass active filter circuit has one pole and one zero on the real axis. The pole and zero are equally distant from and in opposite directions from the origin. A transfer function for this circuit is ##EQU1##
Classically all-pass circuits are fixed value compromise designs that do not provide a good match for both medium and long cable lengths without changing component values. The component values would have to be selected at the field site based upon the cable length. In commercial practice this is not an acceptable procedure. The compromise result, together with the need to custom select components for different length cable sections, suggests that there may be a better way.
Thus there is a problem with the prior art fixed component all-pass active filter circuit. It provides only a single compromise phase equalizer for various cable lengths and does not provide any compensation for phase variations induced by temperature changes experienced by the cable.